Skin structure

Introduction

Skin forms the largest organ of the body – accounting for about 16 per cent of a person's weight.

It performs many vital roles as both a barrier and a regulating influence between the outside world and the controlled environment within our bodies.

Internal body temperature (core temperature) is controlled through several processes, including the combined actions of sweat production and the rate of blood flowing through the network of blood vessels within the skin.

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In cold conditions, skin blood flow drops very low – retaining heat in the centre of the body and preserving the flow of warm blood to the vital organs within the chest and abdomen and to the brain.

Skin and the fat layer beneath it also act as good insulators. When in a hot environment, or when generating heat from muscle activity, skin vessels open up – potentially increasing skin blood flow to as much as a third of the total output of blood from the heart.

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Therefore the skin turns into a heat radiator. The evaporation of sweat from the skin greatly increases the rate at which heat can be lost from the body.

The physical toughness of the skin prevents the ingress of harmful chemicals and invading organisms, such as bacteria and viruses. It also provides resistance to shocks for the more sensitive tissues underneath. At the same time, skin needs to be supple and be able to stretch to accommodate movement.

Skin exposed to sunlight is the main site of manufacture of vitamin D, which is essential for the growth and maintenance of our bones.

The extensive network of nerves, within the skin, feeds information constantly to the brain concerning our surroundings. On the one hand we are warned of harmful extremes of temperature or of other dangers – while on the other, touch can be one of the most powerfully soothing and pleasurable of sensations.

Much importance is attached to the appearance of skin, especially in our modern society.

Medical conditions affecting the skin can have marked effects, not only on our state of well being but also on the ways we interact with other people, on our suitability for certain occupations and on the sorts of pastimes we can enjoy.

Some of the consequences of skin disease, such as rashes and itching, may be obvious and others, such as the psychological impact, can be subtler – although just as important.

The layers of skin

There are two main layers of skin:

epidermis

dermis.

The subcutaneous layer is the fatty layer underneath the skin ('subcutaneous').

It can vary considerably in thickness from person to person depending mostly on whether they are overweight.

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Epidermis

This is the outermost layer. In most parts of the body the epidermis is about 0.1 mm thick, but on the soles of the feet and the palms of the hands it can be 1mm thick or more.

The main skin cell that makes up the epidermis is called the keratinocyte, named this because it produces a tough protein called keratin.

Keratin is also the protein from which nails and hair are formed. It gives skin much of its resistance to physical wear and tear and makes skin waterproof.

Keratinocytes arise in the deepest level of the epidermis and new cells are constantly being produced. As this happens the older cells migrate up to the surface of the skin and eventually are worn off.

On average it takes about 60 days for a new keratinocyte to migrate to the surface and to be shed. Skin scales are the result, and it's perfectly normal for all people to lose about a gram of skin each day in this way. A proportion of the dust that's picked up by your vacuum cleaner is composed of these dead skin cells.

Keratinocytes change in their size and shape from square cells at the base of the epidermis, gradually becoming flatter towards the surface, by which time they have also lost their internal structure.

In healthy skin, these surface cells lie closely together in overlapping fashion, which adds to their protective function.

Dermis

The dermis lies immediately underneath the epidermis and is about four times thicker.

Throughout the dermis other types of protein, notably collagen and elastin, give it strength and flexibility. A reduction in these proteins with age is normal and contributes to the more fragile skin of elderly people.

Medications, in particular steroid drugs, also weaken the collagen fibres – causing thinning of the skin in the long term and an increased tendency to bruising.

Hair arises from root structures, called follicles, which run the whole depth of the dermis.

The hair shaft travels through the epidermis to appear on the skin surface. Along the side of each hair follicle attached is a small gland that produces an oily type of sweat that coats the hair.

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In the deep dermis, there are different types of sweat gland that connect to the surface of the skin through narrow, spiral-shaped sweat ducts – what we normally call the 'pores' of the skin.

Sensation

There are several types of nerve 'sensors' in the skin that have different functions.

The simplest are free nerve endings, and they are similar to bare wires in an electrical circuit. These detect pain, temperature and itch.

Other more complex structures detect pressure or vibration. The distribution of nerves is not even throughout the skin, the highest density being found on the hands, face and genitalia.

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The effect of skin disease in these areas is therefore multiplied by the increased sensitivity of the skin here to sensations arising from the inflammation.

Blood supply

The supply of blood to the skin generally takes the form of microscopically small loops of blood vessels coming up from the deeper layer of larger blood vessels, each to supply a small local area of skin.

The size of the blood vessels in these loops is controlled by signals sent through the nerves that accompany them. Therefore in temperature control, it's possible for them to widen, increasing the flow and so losing excess heat.

If someone is cold or is exposed to a cold environment, contraction of the skin blood vessels cuts the amount of skin blood flow and conserves body heat.

All of this control activity occurs automatically by complex feedback systems involving temperature sensing tissues within the skin and the nervous system and brain.

Immune system and the skin

Apart from the physical barrier protection of the skin, there are specialised cells of the immune system throughout its layers.

Some of these cells detect invasion by foreign proteins, such as bacteria or viruses, and other cells have the function of destroying and removing such material.

Hair and nails

We human beings dispensed with the need for thick body hair long ago in our evolutionary history.

Although head hair provides protection from the potentially damaging effects of ultraviolet light on the skin of the scalp, hair in general has little protective or insulating role in modern man (and woman).

The fact that we still have head hair (or at least most of us do when young) probably relates mostly to its importance in sexual attraction.

Although loss of hair is not a common feature of eczema, involvement of the scalp skin is quite common. Usually it takes the form of excessive scaling (dandruff).

Fungal infections of the scalp are increasingly common in the general population and need to be distinguished from eczematous skin scaling because treatment with anti-fungal shampoo is usually effective.

In eczema, the nails may become shiny from the polishing effect of repeated scratching. But when the eczema itself affects the fingers, the nails can become ridged and thickened.

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Eczema: Eczema is an itchy inflammation of the skin, associated to a varying degree with other features.

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